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What is spring and when does it occur?

Posted on 20 March 2010

What is spring and when does it occur?

According to the astronomical definition, spring occurs when the Sun’s rays strike the equator at noon from directly overhead. This particular time varies from year to year due to variations in the Earth’s orbit about the Sun. In the Northern Hemisphere the vernal (or spring) equinox (equi, “equal,” and nox, “night”) occurs between March 19 and 23, but usually on March 21.

During both equinoxes, all locations on Earth experience 12 hours of daylight and 12 hours of darkness, and the Sun rises due East and sets due West.

Spring has other definitions. The seasons result from the tilt of the Earth and its yearly circling of the Sun. Because spring marks the transition from winter to summer, meteorologists often define spring as the three months from March to May, which means that spring begins on March 1.

We might also define spring as the day on which, if there is precipitation, it is more likely to be rain than snow. For southern Wisconsin, that occurs later in March. We may also define spring based on the appearance of a particular flower, the blooming of certain trees, or the return of specific migrating birds, which are called the phonological signs of spring.

By whatever definition, during spring we’re gaining daylight hours and the air is warming. Welcome news!

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.